In article , PeterC
writes
I'm adding another radiator to a room; it'll be on the outside wall the
same as the present one. The gap will be only about 10cm or so, room to get
valves in.
Instead of fitting valves to both rads., I was wondering about just
connecting them in series with a bit of pipe. Is there any major drawback
to this method?

No bigee, just treat them as one when balancing ie maintain the drop
across the pair rather than each in turn. Technically you will get a
better result if you join both top and bottom with pipes but I don't
have figures on the difference (I'd expect it to be small).
--
fred
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